Thick film materials are mixtures of metal, glass and/or ceramic powders dispersed in an organic vehicle. These materials are applied to nonconductive substrates to form conductive, resistive or insulating films. Thick film materials are used in a wide variety of electronic and light electrical components.
The properties of individual compositions depend on the specific constituents which comprise the compositions. All compositions contain three major components. The conductive phase determines the electrical properties and influences the mechanical properties of the final film. In conductor compositions, the conductive phase is generally a precious metal or mixture of precious metals. In resistor compositions the conductive phase is generally a metallic oxide. In dielectric compositions, the functional phase is generally a glass or ceramic.
The binder of the compositions is usually a glass, a crystalline oxide or a combination of the two. The binder holds the film together and adheres it to the substrate. The binder also influences the mechanical properties of the final film.
The vehicle of the compositions is a solution of polymers in organic solvents. The vehicle determines the application characteristics of the composition.
In the compositions, the functional phase and binder are generally in powder form and are thoroughly dispersed in the vehicle.
Thick film materials are applied to a substrate. The substrate serves as a support for the final film and may also have an electrical function, such as a capacitor dielectric. Substrate materials are generally nonconducting.
The most common substrate materials are ceramics. High-purity (generally 96%) aluminum oxide is the most widely used. For special applications, various titanate ceramics, mica, beryllium oxide and other substrates are used. These are generally used because of specific electrical or mechanical properties required for the application.
In some applications where the substrate must be transparent, such as displays, glass is used.
Thick film technology is defined as much by the processes, as by the materials or applications. The basic thick film process steps are screen printing, drying, and firing. The thick film composition is generally applied to the substrate by screen printing. Dipping, banding, brushing or spraying are occasionally used with irregular shaped substrates. Thick film (TF) resistors and thermistor pastes have been used in the manufacture of ceramic circuit boards. Thermistors are thermally sensitive resistors which have a large temperature coefficient of resistance. They are of two kinds. The first kind exhibits a positive change in resistance with increasing temperature (PTC) and the second kind exhibits a negative change in resistance with increasing temperature (NTC). NTC thermistors ordinarily consist of sintered semiconductive materials and can be used to make elements having resistance values of 10 to 1,000,000 ohms at room temperature. The operational range of such thermistors extends from 75 to 1275K. Therefore, they find extensive use as temperature sensors.
Thermistors are, however, used extensively for such applications as an electronic time delay, capacitor inductor in low frequency oscillators, surge suppressor, voltage or current limiter, gas pressure sensor, thermoconductivity detector, liquid or gas flow sensor and solid or liquid level indicator. Various compositions have been described in the literature for processing in air (oxidizing) atmosphere. The majority of these compositions are based on ruthenium compounds as a conductive phase and lead-cadmium glasses which serve as a binder and need the air (oxidizing) atmosphere.
Conductive phases such as LaB.sub.6 and doped tin oxide have been disclosed for processing in inert and reducing atmospheres. These compositions contain lead and Cd free glasses. However, these compositions cannot be processed in air because the conductive phases oxidize in air. The oxidation in air render these conductive phases insulators (LaB.sub.6) and inappropriate (doped SnO.sub.2) for resistor use, due to intrinsic change in electrical properties.
Resistive pastes of the prior art tend to have high positive temperature coefficient of resistance (TCR) at low resistance; usually at .gtoreq.10.OMEGA./.quadrature./mil. To lower the TCR of these resistive pastes it is common to add TCR drivers to them. TCR drivers such as manganese oxide, Nb.sub.2 O.sub.5 and TiO.sub.2 to lower the TCR, however, they also raise the resistance. To compensate for the increase in resistance it is a common practice to add more conductive phase. As a result of TCR and resistance optimization the prior art low resistance pastes tend to have high volume fraction of conductive phase and less vitreous phase. This process impacts the stability of low resistance pastes and they are less stable than mid range 100.OMEGA./.quadrature.-100 k .OMEGA./.quadrature. resistive pastes.
TF thermistor compositions especially of the NTC (negative temperature coefficient of resistance) type have a larger TCR at low resistances. Moreover, it is difficult to make low resistance NTC pastes; U.S. Pat. No. 5,122,302 to J. Hormadaly discloses NTC type thick film thermistor pastes in which it is shown that TCR increases when R decreases and only the 1 k .OMEGA./.quadrature.-1M .OMEGA./.quadrature. range is disclosed.
Nontoxic lead-free and cadmium-free frit systems which are low melting, moderate in expansion and durability that provide excellent wetting are not, however, known in the art for use in the above composition. Therefore, consistent with efforts to reduce or eliminate lead and cadmium from broad categories of products containing glass frits, the present invention deals with a lead-free and cadmium-free glass frit that has been shown to be useful in the formulation of thick film paste compositions. The composition of the present invention renders a nontoxic, cadmium-free/lead-free thick film alternative to presently used resistor and thermistor compositions.